Unstable moisture from Tropical Storm Ivo is being sucked into Southern California, where it will produce potentially severe thunderstorms and rain across San Diego County on Sunday and Monday, says the National Weather Service. A flash flood advisory has been issued for East County, with special attention being paid to the area near Julian where the Chariot fire burned more than 7,000 acres of wildland in July.

Moisture from Ivo and the monsoon produced thunder clouds above Big Black Mountain near Ramona on Saturday. HPWREN

Moisture from Ivo and the monsoon produced thunder clouds above Big Black Mountain near Ramona on Saturday.

Air is flowing clock-wise around a big high pressure system centered over Missouri, drawing moisture up from the Gulf of Mexico. At the same time, air is flowing counter-clockwise around a low pressure system off British Columbia, grabbing some of the same moisture. The systems are basically twin magnets that are claiming the remnants of Ivo, which is falling apart southwest of Cabo San Lucas.

"The moisture could produce thunderstorms in the county's deserts and mountains on Sunday, and cause scattered rain and thunderstorms all the way to the coast on Monday, when the main surge arrives," said Joe Dandrea, a weather service forecaster.

The thunderstorms could become strong enough to generate a half-inch or more of rain in some areas, especially East County. The tropical air also will cause the entire region to become humid late Sunday and Monday.

A new tropical depression is forming off the Pacific coast of Mexico and could influence San Diego's weather next Thursday and Friday.